Genome-Wide Association Mapping of Bacterial Leaf Streak Resistance in Two Elite Barley Breeding Panels.

Bacterial leaf streak (BLS), caused chiefly by the pathogen Xanthomonas translucens pv. translucens, is becoming an increasingly important foliar disease of barley in the Upper Midwest. The deployment of resistant cultivars is the most economical and practical method of control. To identify sources of BLS resistance, we evaluated two panels of breeding lines from the University of Minnesota (UMN) and Anheuser-Busch InBev (ABI) barley improvement programs for reaction to strain CIX95 in the field at St. Paul and Crookston, MN, in 2020 and 2021. The percentage of resistant lines in the UMN and ABI panels with mid-season maturity was 1.8% (6 of 333 lines) and 5.2% (13 of 251 lines), respectively. Both panels were genotyped with the barley 50K iSelect SNP array, and then a genome-wide association study was performed. A single, highly significant association was identified for BLS resistance on chromosome 6H in the UMN panel. This association was also identified in the ABI panel. Seven other significant associations were detected in the ABI panel: two each on chromosomes 1H, 2H, and 3H and one on chromosome 5H. Of the eight associations identified in the panels, five were novel. The discovery of resistance in elite breeding lines will hasten the time needed to develop and release a BLS-resistant cultivar.

Genetic dissection of a pericentromeric region of barley chromosome 6H associated with Fusarium head blight resistance, grain protein content and agronomic traits.

KEY MESSAGE: Fine mapping of barley 6H pericentromeric region identified FHB QTL with opposite effects, and high grain protein content was associated with increased FHB severity. Resistance to Fusarium head blight (FHB), kernel discoloration (KD), deoxynivalenol (DON) accumulation and grain protein content (GPC) are important traits for breeding malting barley varieties. Previous work mapped a Chevron-derived FHB QTL to the pericentromeric region of 6H, coinciding with QTL for KD resistance and GPC. The Chevron allele reduced FHB and KD, but unfavorably increased GPC. To determine whether the correlations are caused by linkage or pleiotropy, a fine mapping approach was used to dissect the QTL underlying these quality and disease traits. Two populations, referred to as Gen10 and Gen10/Lacey, derived from a recombinant near-isogenic line (rNIL) were developed. Recombinants were phenotyped for FHB, KD, DON, GPC and other agronomic traits. Three FHB, two DON and two KD QTLs were identified. One of the three FHB QTLs, one DON QTL and one KD QTL were coincident with the GPC QTL, which contains the Hv-NAM1 locus affecting grain protein accumulation. The Chevron allele at the GPC QTL increased GPC and FHB and decreased DON and KD. The other two FHB QTL and the other DON and KD QTL were identified in the regions flanking the Hv-NAM1 locus, and the Chevron alleles decreased FHB, DON and KD. Our results suggested that the QTL associated with FHB, KD, DON and GPC in the pericentromeric region of 6H was controlled by both pleiotropy and tightly linked loci. The rNILs identified in this study with low FHB severity and moderate GPC may be used for breeding malting barley cultivars.

Rpg7: A New Gene for Stem Rust Resistance from Hordeum vulgare ssp. spontaneum.

Wheat stem rust (causal organism: Puccinia graminis f. sp. tritici) is an important fungal disease that causes significant yield losses in barley. The deployment of resistant cultivars is the most effective means of controlling this disease. Stem rust evaluations of a diverse collection of wild barley (Hordeum vulgare ssp. spontaneum) identified two Jordanian accessions (WBDC094 and WBDC238) with resistance to a virulent pathotype (P. graminis f. sp. tritici HKHJC) from the United States. To elucidate the genetics of stem rust resistance, both accessions were crossed to the susceptible landrace Hiproly. Segregation ratios of F2 and F3 progeny indicated that a single dominant gene confers resistance to P. graminis f. sp. tritici HKHJC. Molecular mapping of the resistance locus was performed in the Hiproly/WBDC238 F2 population based on 3,329 single-nucleotide polymorphism markers generated by genotyping-by-sequencing. Quantitative trait locus analysis positioned the resistance gene to the long arm of chromosome 3H between the physical/genetic positions of 683.8 Mbp/172.9 cM and 693.7 Mbp/176.0 cM. Because this resistance gene is novel, it was assigned the new gene locus symbol of Rpg7 with a corresponding allele symbol of Rpg7.i. At the seedling stage, Rpg7 confers resistance against a number of other important P. graminis f. sp. tritici pathotypes from the United States (MCCFC, QCCJB, and TTTTF) and Africa (TTKSK) as well as an isolate (92-MN-90) of the rye stem rust pathogen (P. graminis f. sp. secalis) from Minnesota. The resistance conferred by Rpg7 can be readily transferred into breeding programs because of its simple inheritance and clear phenotypic expression.

Cold Conditioned: Discovery of Novel Alleles for Low-Temperature Tolerance in the Vavilov Barley Collection.

Climate changes leading to higher summer temperatures can adversely affect cool season crops like spring barley. In the Upper Midwest region of the United States, one option for escaping this stress factor is to plant winter or facultative type cultivars in the autumn and then harvest in early summer before the onset of high-temperature stress. However, the major challenge in breeding such cultivars is incorporating sufficient winter hardiness to survive the extremely low temperatures that commonly occur in this production region. To broaden the genetic base for winter hardiness in the University of Minnesota breeding program, 2,214 accessions from the N. I. Vavilov Institute of Plant Industry (VIR) were evaluated for winter survival (WS) in St. Paul, Minnesota. From this field trial, 267 (>12%) accessions survived [designated as the VIR-low-temperature tolerant (LTT) panel] and were subsequently evaluated for WS across six northern and central Great Plains states. The VIR-LTT panel was genotyped with the Illumina 9K SNP chip, and then a genome-wide association study was performed on seven WS datasets. Twelve significant associations for WS were identified, including the previously reported frost resistance gene FR-H2 as well as several novel ones. Multi-allelic haplotype analysis revealed the most favorable alleles for WS in the VIR-LTT panel as well as another recently studied panel (CAP-LTT). Seventy-eight accessions from the VIR-LTT panel exhibited a high and consistent level of WS and select ones are being used in winter barley breeding programs in the United States and in a multiparent population.

Genome-wide association analysis of natural variation in seed tocochromanols of barley.

Tocochromanols (tocols for short), commonly called Vitamin E, are lipid-soluble plant antioxidants vital for regulating lipid peroxidation in chloroplasts and seeds. Barley (Hordeum vulgare L.) seeds contain all eight different isoforms of tocols; however, the extent of natural variation in their composition and their underlying genetic basis is not known. Tocol levels in barley seeds were quantified in diverse H. vulgare panels comprising 297 wild lines from a diversity panel and 160 cultivated spring-type accessions from the mini-core panel representing the genetic diversity of the USDA barley germplasm collection. Significant differences were observed in the concentration of tocols between the two panels. To identify the genes associated with tocols, genome-wide association analysis was conducted with single nucleotide polymorphisms (SNPs) from Illumina arrays for the mini-core panel and genotyping-by-sequencing for the wild barley panel. Forty unique SNPs in the wild barley and 27 SNPs in the mini-core panel were significantly associated with various tocols. Marker-trait associations (MTAs) were identified on chromosomes 1, 6, and 7 for key genes in the tocol biosynthesis pathway, which have also been reported in other studies. Several novel MTAs were identified on chromosomes 2, 3, 4 and 5 and were found to be in proximity to genes involved in the generation of precursor metabolites required for tocol biosynthesis. This study provides a valuable resource for barley breeding programs targeting specific isoforms of seed tocols and for investigating the physiological roles of these metabolites in seed longevity, dormancy, and germination.

Improving Prediction Accuracy Using Multi-allelic Haplotype Prediction and Training Population Optimization in Wheat.

The use of haplotypes may improve the accuracy of genomic prediction over single SNPs because haplotypes can better capture linkage disequilibrium and genomic similarity in different lines and may capture local high-order allelic interactions. Additionally, prediction accuracy could be improved by portraying population structure in the calibration set. A set of 383 advanced lines and cultivars that represent the diversity of the University of Minnesota wheat breeding program was phenotyped for yield, test weight, and protein content and genotyped using the Illumina 90K SNP Assay. Population structure was confirmed using single SNPs. Haplotype blocks of 5, 10, 15, and 20 adjacent markers were constructed for all chromosomes. A multi-allelic haplotype prediction algorithm was implemented and compared with single SNPs using both k-fold cross validation and stratified sampling optimization. After confirming population structure, the stratified sampling improved the predictive ability compared with k-fold cross validation for yield and protein content, but reduced the predictive ability for test weight. In all cases, haplotype predictions outperformed single SNPs. Haplotypes of 15 adjacent markers showed the best improvement in accuracy for all traits; however, this was more pronounced in yield and protein content. The combined use of haplotypes of 15 adjacent markers and training population optimization significantly improved the predictive ability for yield and protein content by 14.3 (four percentage points) and 16.8% (seven percentage points), respectively, compared with using single SNPs and k-fold cross validation. These results emphasize the effectiveness of using haplotypes in genomic selection to increase genetic gain in self-fertilized crops.

Genetic architecture of agronomic and quality traits in a nested association mapping population of spring wheat.

Germplasm collections are rich sources of genetic variation to improve crops for many valuable traits. Nested association mapping (NAM) populations can overcome the limitations of genome-wide association studies (GWAS) in germplasm collections by reducing the effect of population structure. We exploited the genetic diversity of the USDA-ARS wheat (Triticum aestivum L.) core collection by developing the Spring Wheat Multiparent Introgression Population (SWMIP). To develop this population, twenty-five core parents were crossed and backcrossed to the Minnesota spring wheat cultivar RB07. The NAM population and 26 founder parents were genotyped using genotyping-by-sequencing and phenotyped for heading date, height, test weight, and grain protein content. After quality control, 20,312 markers with physical map positions were generated for 2,038 recombinant inbred lines (RILs). The number of RILs in each family varied between 58 and 96. Three GWAS models were utilized for quantitative trait loci (QTL) detection and accounted for known family stratification, genetic kinship, and both covariates. GWAS was performed on the whole population and also by bootstrap sampling of an equal number of RILs from each family. Greater power of QTL detection was achieved by treating families equally through bootstrapping. In total 16, 15, 12, and 13 marker-trait associations (MTAs) were identified for heading date, height, test weight, and grain protein content, respectively. Some of these MTAs were coincident with major genes known to control the traits, but others were novel and contributed by the wheat core parents. The SWMIP will be a valuable source of genetic variation for spring wheat breeding.

Genome-Wide Association Mapping of Stem Rust Resistance in Hordeum vulgare subsp. spontaneum.

Stem rust was one of the most devastating diseases of barley in North America. Through the deployment of cultivars with the resistance gene Rpg1, losses to stem rust have been minimal over the past 70 yr. However, there exist both domestic (QCCJB) and foreign (TTKSK aka isolate Ug99) pathotypes with virulence for this important gene. To identify new sources of stem rust resistance for barley, we evaluated the Wild Barley Diversity Collection (WBDC) (314 ecogeographically diverse accessions of Hordeum vulgare subsp. spontaneum) for seedling resistance to four pathotypes (TTKSK, QCCJB, MCCFC, and HKHJC) of the wheat stem rust pathogen (Puccinia graminis f. sp. tritici, Pgt) and one isolate (92-MN-90) of the rye stem rust pathogen (P. graminis f. sp. secalis, Pgs). Based on a coefficient of infection, the frequency of resistance in the WBDC was low ranging from 0.6% with HKHJC to 19.4% with 92-MN-90. None of the accessions was resistant to all five cultures of P. graminis A genome-wide association study (GWAS) was conducted to map stem rust resistance loci using 50,842 single-nucleotide polymorphic markers generated by genotype-by-sequencing and ordered using the new barley reference genome assembly. After proper accounting for genetic relatedness and structure among accessions, 45 quantitative trait loci were identified for resistance to P. graminis across all seven barley chromosomes. Three novel loci associated with resistance to TTKSK, QCCJB, MCCFC, and 92-MN-90 were identified on chromosomes 5H and 7H, and two novel loci associated with resistance to HKHJC were identified on chromosomes 1H and 3H. These novel alleles will enhance the diversity of resistance available for cultivated barley.